Utility grid companies set out the strategies and requirements for the connection of wind turbines to the utility grid. These connection requirements are described in so-called grid codes. The grid codes vary depending upon the geographical location of the utility grid.
One of the topics discussed in grid codes is the capabilities of a wind turbine when the utility grid experiences a fault. One requirement may be that the wind turbine stay connected and synchronized to the utility grid during the grid fault, at least for some types of faults.
When a wind turbine experiences a utility grid fault the generator speed increases almost immediately as a result of the excessive aerodynamic power that cannot be converted to electrical power. Therefore the aerodynamic power must be reduced drastically throughout the period of the utility grid fault. During the fault condition the turbine is operated in a fault mode.
Upon recovery of the utility grid, the wind turbine needs to recover from the fault mode and resume normal operation.
It is against this background that the invention has been devised.